1. Preheat the oven to 425 degrees. Mist two 8-inch round cake pans with cooking spray.
2. Stir in the buttermilk, oil and egg whites until just combined. Distribute the batter evenly between the pans; sprinkle with the cheddar and scallions.
3. Bake 15 to 18 minutes or until a toothpick inserted into the center comes out clean. Let cool and then slice into squares and serve.